FACIES-CONTROLLED SHRINKAGE-CRACK ASSEMBLAGES IN MIDDLE PROTEROZOIC MUDSTONES FROM MONTANA, USA

Shrinkage‐crack morphotypes in the Libby Formation (upper Belt Supergroup) are confined to distinct environmental facies. The lower facies is characterized by flat rip‐up clasts, stromatolites, oolites, small‐scale symmetrical ripples, and fenestral fabric. These rocks were deposited above fair‐weather wave base on a periodically exposed mudflat. Shrinkage cracks in this facies are predominantly branching, incompletely connected features in plan view, except for local examples of completely connected polygonal cracks on purple argillite bed tops and rare, long spindle‐shaped cracks on bed tops of dark grey argillite. The upper facies was deposited below fair‐weather wave base and contains mainly unconnected, short spindle‐shaped shrinkage cracks, and rare slightly branching cracks. Restriction of some crack types to certain facies better constrains interpretation of the origin of these shrinkage cracks. The cracks in the upper facies were strongly influenced by sediment loading, and may have formed by compaction‐induced expulsion of water from pore space, resulting in synaeresis cracks. In the underlying shallower facies, polygonal cracks formed by desiccation. Elsewhere in this facies, incomplete, partially connected cracks and long spindle‐shaped cracks on the same bedding plane are interpreted as having formed by desiccation. Shrinkage cracks are an under‐used source of environmental information, but confusion as to their origin sometimes restricts their potential. More intensive analysis of properties of host sediment and crack fills may further our understanding of depositional and diagenetic influence on crack morphology. Crack cross‐sections, which are often more commonly exposed than bedding plane cracks, may provide critical additional information on crack genesis. Better understanding of crack genesis will strengthen our ability to interpret unfossiliferous muddy sequences common in Precambrian and lacustrine settings. Copyright © 1990, Wiley Blackwell. All rights reserved